Various silicon-based devices, such as a Micro-Electro-Mechanical Systems (MEMS) gyroscope, are mechanically coupled to a substrate. Coupling of the MEMS device to the substrate occurs at one or more anchor points (anchors) bonded to the substrate, referred to herein as mesas.
Portions of the MEMS device may be suspended from the substrate using one or more silicon flexures. A number of recesses etched into the substrate allow the portions of the MEMS device, referred to herein as a MEMS mechanism, to move freely within an interior portion of the MEMS device.
FIG. 1 is a top view 100 of a prior art anchor 102 bonded to a portion of a substrate 104. FIG. 2 is a side cut-away view 200 of the prior art anchor 102 bonded to the portion of a substrate 104. The anchor 102 is bonded to the substrate 104 on a mesa 106 along the contact region 108 where the anchor 102 and the substrate 104 are in contact.
The anchor 102 fixes and supports a MEMS mechanism 118 via an interconnecting flexure 110 or the like. Flexure 110 allows for movement of the MEMS mechanism 118 in selected directions and limits movement in other directions.
Prior to bonding the anchor 102 to the substrate 104, a trace 112 is formed from an external location to a location on the mesa 106. The trace 112 may be made of a metal and formed by a suitable process, such as metal sputtering over a mask oriented above the substrate 104.
An optional bump 114 (or bumple) is located at or near the end of the trace 112 to facilitate electrical coupling between the anchor 102 and the trace 112. If the trace 112 is made of metal, the bump 114 may be the same metal or another metal that is relatively soft and deformable under pressure and/or temperature.
Often, the process of bonding the anchor 102 to the substrate mesa 106 uses a pressure which deforms the trace 112 and/or the bump 114. Deformation of the trace 112 and/or the bump 114 improves the electrical connectivity between the anchor 102 and the trace 112.
During the process of bonding, stresses occur in the anchor 102. The stress is caused by forces exerted by the trace 112 and/or the bump 114 onto the material of the anchor 102. Such stresses (conceptually illustrated by a plurality of stress lines 116) exerted by the bump 114 to the anchor 102 may cause damage or otherwise stress the material of the anchor 102. Further, temperature fluctuations during use of the MEMS device may change the relative size of the trace 112 and/or the bump 114, inducing a time-varying change in the stress of the material of the anchor 102. Additional stress may be induced by a coefficient of thermal expansion mismatch between silicon and substrate material.
The stresses induced in the material of the anchor 102 are undesirable in that such stresses, in addition to forces transmitted from the flexure 110 to the anchor 102 during MEMS device use, may result in the formation of microcracks. Such microcracks may lead to structure failure of the MEMS device at the anchor 102.
Additionally, the movement of the MEMS mechanism 118 generates forces that are transmitted to the anchor 102 via the flexure 110. The stresses and/or microcracks may sufficiently weaken the anchor 102 such that the anchor 102 may structurally fail when the forces generated by movement of the MEMS mechanism 118 are transmitted to the anchor 102.
Accordingly, the anchor 102 is designed with sufficient size and mass to accommodate the stresses induced in the material of the anchor 102 from the trace 112 and/or the bump 114, and to accommodate forces generated by the MEMS mechanism 118. However, the prior art process has several disadvantages. The minimum size of the anchor 102 is limited since the anchor 102 must have sufficient material to accommodate the induced stresses. As MEMS devices become increasingly smaller, it is very desirable to reduce the size of anchors used in a MEMS device. Further, it is desirable to reduce fabrication process costs.